The present invention relates to a valid patch discrimination method for an automatic density control apparatus.
Normally, in density control of a print, a color patch in a color bar (control strip) printed on a margin portion of printed matter is measured by a densitometer, and a degree of opening of an ink key or an ink amount of a printing press is manually or automatically controlled based on the measured density. In recent years, an automatic density control apparatus has been proposed wherein a color bar is measured by a scanning type densitometer, the measured density is compared with a prestored reference density, and the degree of opening of an ink key of a printing press is automatically controlled.
However, a variety of sizes of paper are used in printing, and paper smaller than maximum paper for a printing press is frequently used for printing. For this reason, when a density is measured by the scanning type densitometer, a valid color patch is determined by the following methods, and density measurement and control are performed. In a first method, an operator inputs a paper size or nonused ink key, and a measurement/control range is determined based on the input data. In a second method, a measurement start patch is printed in a color bar, and the measurement/control range is determined according to the position of the patch.
However, the first method applies a load to the operator. In the second method, the measurement start patch must be printed for each paper size, and another patch corresponding to the printed patch must be deleted. A color may be corrected due to temporary contamination or omission (a color patch or its part is not printed due to a process error or contamination on a plate) although it need not be corrected.